There is a pressing need to more efficiently produce electric power from alternative sources and methods. While carbon based fuels have dominated energy generation in the past, there is growing interest in sources and methods that provide less pollution and higher efficiency. The direct conversion of heat into electric current flow, if made efficient and practical, opens the possibility of a wide range of heat sources to drive a Thermionic Energy Conversion (TEC) system, from conventional fuels, to solar concentrators, to geothermal or any other heat source, including reclaimed heat.
Thermionic energy conversion (TEC) is a technique that allows for the efficient conversion of thermal energy directly into electrical energy [1-6]. TEC is based on the widely understood physical principal of thermionic emission which describes the thermal emission of electrons from a heated cathode, relative so the anode, as shown in FIG. 1. As a cathode is heated above zero Kelvin, it can be predicted, based on Fermi-Dirac statistics that some of the cathode's electrons will have energies equal to or greater than the cathode's work function. The work function is the energy required for an electron to be emitted into a vacuum. This process can be described by the Richardson Equation (Equation 1) [7-8].J=AT2e(−Φ/kT)  (1)
Where: J=Current Density (A/cm2); A=Richardson Constant (A/K2 cm2); T=Temperature (K); Φ=work function (eV); and k=Boltzmann constant (eV/K). It follows from the Richardson equation that high thermionic emission current densities can be achieved by a material with a high Richardson constant and a low work function.
A thermionic energy conversion (TEC) system comprises a cathode, an anode, a controlled environment between the two, and the necessary electrical connections to enable the current generated to flow in an external circuit. In a basic thermionic converter, the cathode and anode are separated by a gap which is generally in a vacuum and which enables electrons to cross without intercepting (i.e. colliding with) gas molecules or ions. There exists prior art that incorporates gaseous species into this gap at relatively low concentrations to enhance electron emission from the cathode. As thermal energy is imparted to the cathode, electrons with sufficient energy will emit thermionically from the surface and traverse the vacuum gap where they collect at the anode. The electrons then provide energy to an electrical load as they are cycled back to the cathode through an electrical circuit between anode and cathode.
The prior art suggests that diamond is an ideal material for the cathode in a TEC system. Diamond has unique properties that make it especially suited for this purpose [9].
Diamond has a wide band gap, 5.5 eV and, when doped, will become electrically conductive, and its conductivity will increase at elevated temperatures. In one embodiment, doped diamond polycrystalline film is grown in an environment with boron; in another embodiment it is grown with nitrogen. Establishing a low resistance path for electrical current utilizing such doped diamond material is detailed in the prior art, and has been demonstrated [10].
Diamond material maintains its physical integrity at very high temperatures (e.g. up to the range of 1000-1200 degrees C.) because of the strength of the carbon sp3 bonding and has the ability to withstand repetitive cycling from an ambient of approximately room temperature to high (e.g. 1000 degrees C.), as well as low (negative 100 degrees C.) temperatures. The compactness of the atomic structure prevents typical doping ions (e.g. boron) from out-gassing (out-diffusing), or decreasing in concentration at high temperatures. An additional advantage of the robustness of the diamond crystal lattice is its virtual immunity to radiation damage and other forms of environmental stress [11].
Importantly, diamond or certain material containing diamond has a very low work-function and low electron affinity, which makes the emission of electrons from said diamond surface more efficient than with most other materials [12].
In addition, diamond has the highest thermal conductivity of any known material, approximately five times that of copper, and therefore the design of systems in which heat is readily conducted to the electron-emitting surface, or extracted from the anode, is simplified and made more efficient [13].
Chemical Vapor Deposited (CVD) polycrystalline diamond has nearly all of the superior material properties of single crystal diamond without the high cost. In addition, it can be patterned and deposited and doped into a semiconductor, and processed with many known silicon semiconductor processing methods. Diamond and such diamond films can be made substantially conductive by incorporating nitrogen, boron or other dopant materials in its growth.
Diamond has the rare combination of material properties of extremely high thermal conductivity and the control of electrical conductivity: i.e. can be fabricated with known methods by addition of other materials in small concentrations (doping), resulting in a polycrystalline diamond film with high electrical conductivity.
Therefore, it first appears that diamond would make an ideal electron emitter in TEC systems, following the Richardson equation to very high temperatures; in practice, diamond cathode emitters have a limitation. Such emitters have consistently shown enhanced emission to approximately 600-800 degrees Centigrade, at which point electron emission begins to diminish, and as temperature is further increased, electron emission decreases approaching zero.
Recent prior art [1] suggests the introduction of certain gas species, such as hydrogen (or gas molecules containing hydrogen), into the vacuum chamber between the cathode and anode have demonstrated increases in emission current. The concentration of gas that can be introduced into the gap is limited by the fact that if too high, a substantial percentage of electrons crossing from cathode to anode will suffer a collision with a gas molecule or ion, and will fail to transport. The previously mentioned reference [1] discloses the introduction of hydrogen-containing gas species into the gap while maintaining a vacuum at or below 5.5×10−6 Torr, which remains a relatively low concentration of hydrogen in said gap.
It has been reported that the exposure of diamond cathodes to a low energy hydrogen plasma enhances the thermionic emission current from diamond films [14]. This enhancement is reviewed and extensively described in Reference [1]. That is, it has been documented by multiple sources, as is further summarized in Reference [1], that a diamond layer heated in a partial pressure of hydrogen (or certain hydrogen bearing gaseous species) will emit electrons somewhat more efficiently under certain conditions, but there is no prior art demonstrating the potential to significantly increase the electric current emitted to an anode to a substantially higher value than that of a pure vacuum, thereby providing a practical level of electric power generation. [1, 9-14] Thus, trying to provide an atmosphere with a partial pressure of hydrogen in the gap between cathode and anode, exposing the diamond cathode emitter surface will not prolong or preserve emission at higher temperatures, i.e., at high temperature (e.g. above about 600-800 degrees Centigrade) because the hydrogen or hydrogen ions cannot reside on the diamond surface to provide the electron escape enhancement.
The method of reference [1] has only demonstrated improvement in electron emission and related efficiency in the range of 10 percent or less. Thus, a significant innovation is required in order to achieve increased current density at temperatures well above the range of 600-800 degrees Centigrade and extending to above 1000 degrees Centigrade. This is the subject of the present invention.
In order to more quantitatively define the above limitation, we refer to the Richardson equation (Equation 1) which describes the ideal performance of thermionic electron emission. As shown in FIG. 2, the solid curve is a plot of the Richardson equation for a diamond emitter, and projects a current increasing super-linearly with temperature, reaching significant currents at high temperatures. Extrapolation of this curve to temperatures in the range of 900-1100 degrees Centigrade predicts unprecedented current production per unit area. The present reality is that shown with the dots in FIG. 2, in which the current peaks at a temperature in the range of 600 to 800 degrees Centigrade, and then decreases. The method of introducing a partial pressure of hydrogen or hydrogen ions, or hydrogen containing compounds into the said gap results in only a modest improvement in electron emission.
If this limitation of TEC efficiency at temperatures above 700 degrees Centigrade could be overcome, then this technology can approach total energy conversion efficiencies of 90% of the Carnot limit, which is a vast improvement over current technologies. This invention addresses eliminating the previously mentioned limitation and enables TEC devices to perform at significantly higher temperatures and with a corresponding improvement in current output per unit area of emission. This invention enables the practical direct thermal generation of electrical power with the TEC approach. [1-5]